A command given to someone who needs to get his or her shit together. In a sports context, often said to one whose head isn't in the game, or who is playing erratically. In social situations, said to someone who is acting really strange or being embarrassing.

Joe: "Shit, this bar is never this crowded on a Wednesday... what are all these girls doing here? Looks like we picked a good night to come out."
Dwight: "Damn, look at the tits on these girls over here."
Hot Girl: "I heard that, you fucking creep."
{Joe angrily pulls Dwight aside.}
Joe: {pointing his finger in Dwight's face} "Hey, look at me. You lock it up. Don't be a liability tonight."
Dwight: "Alright, alright. Honest mistake."